1. Field of the Invention
The present invention relates generally to a method and apparatus for matching a pacer to a patient's heart, and more particularly to a method and apparatus for matching the sensitivity threshold of a pacer's sense amplifier with the effective cardiac signal strength of a patient's heart. The system and method are particularly well adapted for use in a pacer system analyzer, wherein the operation of a pacer is monitored in association with a patient's heart prior to implantation.
2. Description of the Prior Art
To assist physicians in treating cardiac disorders of the type for which the use of implantable cardiac pacers is indicated, pacer system analyzers (PSA's) have been developed. These devices are used at the time of pacer implantation to efficiently measure the parameters of a pacer system, including a patient's heart, a pacer and implanted pacer leads, without the need to perform separate procedures requiring multiple interconnections and an undesirably long time to complete. Pacers to be implanted are tested for proper programming and operation, not only while connected in a simulated pacing system environment, but also while operating in the actual system in which they are to be used. Moreover pacer system analyzers are preferably equipped to generate pacing pulses as required to support the patient during the pacer implantation process, independently of the pacer to be implanted.
By using a pacer system analyzer, a physician is able to adjust the operating parameters of a pacer system as required to suit the specific needs of an individual patient before the pacer has been fully implanted and the implantation surgery completed. This minimizes the need for inconvenient and potentially injurious post-implantation adjustment of the pacer or its associated pacer leads.
One important parameter of a pacer system is the sensing margin of the pacer to cardiac activity as sensed by the pacer leads. It is desirable that the pacer be sufficiently sensitive to reliably recognize and respond to the activity of the heart, but not so sensitive as to respond to extraneous noise or motion artifacts inherently present in the pacer lead system.
In prior pacer system analyzer constructions the peak or other voltage level of the cardiac pulse as sensed at the cardiac pacer lead was first measured, and the sensitivity of the pacer was then set so that the pacer responded to signals of that voltage level. Unfortunately, this did not always provide reliable sensing because of differing frequency spectrum characteristics of the pacer, the PSA, and the cardiac signal. Consequently, the physician was forced to either set the sensitivity of the pacer undesirably high, thereby risking response to noise and motion artifacts, or to set the pacer sensitivity at the cardiac signal level, thereby risking having to post-operatively alter the placement of the pacer lead or adjust pacer sensitivity in the event of inadequate cardiac signal. The present invention is directed to a measurement system and method for use in a pacer system analyzer which avoids this problem through pre-implantation use of a novel user-controllable broad-band attenuator element between an operatively connected cardiac lead and a pacer to establish a true sensing margin for the implanted pacer and lead combination.
In operation, in previous pacer system analyzers the terminals of an implanted pacer lead were first connected to a surrogate sensing amplifier within the analyzer. This sensing amplifier had a predetermined sensitivity calibration with a specific test signal and a predetermined frequency response, which was designed for a "typical heart" and which was considered a "standard". Then, with the analyzer, the physician determined the level of signal picked up from the patient's heart and matched it with a pacer having a somewhat higher bench tested sensitivity.
However, the sensing amplifier in the pacer did not necessarily have the same calibration as the surrogate sensing amplifier in the PSA, and more particularly, did not necessarily have the same frequency response characteristics. As a result, the pacer, when connected to the patient's heart, did not necessarily have the same effective sensitivity as the surrogate test sensing amplifier. Accordingly, the pacer connected to the patient's heart was not necessarily able to sense low level cardiac signals from the patient's heart, or worse yet, would marginally sense the patient's heart signal.
Heretofore various devices have been proposed for matching a pacer to a patient's heart and examples of such previously proposed devices are disclosed in the following U.S. patents:
______________________________________ U.S. Pat. No. PATENTEE ______________________________________ 3,757,790 Herrmann 3,777,762 Nielsen 3,800,801 Gaillard 4,245,643 Benzing, III et al 4,290,430 Bihn et al 4,337,776 Daly et al ______________________________________
The Hermann U.S. Pat. No. 3,757,790 is directed to a capture threshold analyzer and stimulating testing device with an internal generator. The circuitry used is complicated and the device does not appear to be adapted for in vivo cardiac signal sensing analysis.
The Nielsen U.S. Pat. No. 3,777,762 discloses only a pacemaker output control circuit.
The Gaillard U.S. Pat. No. 3,800,801 is directed to a heart stimulation apparatus and method of testing its installation including means for attenuating the pacing and separate means for picking up calibrated pulses and displaying same.
The Benzing, III, et al. U.S. Pat. No. 4,245,643 discloses an apparatus for measuring contact resistance of a pacing electrode.
The Bihn, et al. U.S. Pat. No. 4,290,430 is directed to a pacer analyzer wherein an analog signal is converted to a digital signal and then back to an analog signal. The pacer is not connected directly to the heart in this analyzer. Rather, pulses from the heart are digitized, stored and then analog conversion thereof is applied to the pacer.
Although the circuitry disclosed in the Bihn, et al. patent attempts to achieve a function similar to the function of the sense margin evaluator of the present invention, it does so in a different and more complicated fashion. In this respect, the sense margin evaluation system of the present invention utilizes in-vivo real time evaluation of a heart signal. In contrast, the circuitry disclosed in the Bihn, et al. patent uses a microprocessor system to do an A/D conversion followed by storage and subsequent D/A playback with a scaling factor injected into the procedure.
The Daly, et al. U.S. Pat. No. 4,337,776 is directed to an impedance measuring pacer which facilitates measurement of both electrode impedance and stimulation threshold.
As will be described in greater detail hereinafter, instead of comparing the sensitivity of the pacer sense amplifier with a "test" standard and measuring sensitivity, which is frequency dependent and which can vary for different sensing amplifiers in different pacers and in different PSA sensing amplifiers because of different frequency response characteristics, the sense margin evaluation system of the present invention provides for coupling of the implanted cardiac lead of a cardiac pacing system to a broad-band variable attenuator having its output connected to the system pacer, so that a meaningful matching of a particular pacer to a particular heart is obtained.
With the sense margin evaluation system of the present invention, instead of specifying different levels of sensivity in millivolts with respect to a test signal, the ratio, or "sense margin", of effective cardiac signal strength relative to the actual sensing amplifier threshold is measured and specified. This ratio will preferably be between 2:1 and 5:1 and can, for example, be 4:1.